The Evolution of Viviparity in Amphibians: A Study of Homoplasy

胎生是一种活生生的生殖方式，而不是产卵方式，在许多动物的谱系中都是独立进化的。因此，这是趋同进化的结果，相同的特征出现在不共享带有该特征的共同祖先(同质性)的分类群中。两栖动物的胎产对这些独立的进化有许多不同的表现，包括内部受精的卵子保留在输卵管中，在卵黄被吸收后没有母体营养，输卵管内自相残杀，外部受精的卵子保留在母亲背部的皮肤中，等等。本研究计划将通过比较1)亲本卵巢、输卵管和皮肤的特征与近缘产蛋物种的卵巢、输卵管和皮肤的特征来阐明‘妊娠’维持的影响；2)具有不同胎生模式的类群和相关产卵物种的胚胎的发育模式；3)发育中的胚胎的保留模式，包括从形态上测量的激素底物；4)发育中的幼虫的营养模式；5)胎生物种幼虫的变态特征与相关蛋鸡的幼虫的变态特征；6)“孕期”的长短以及分娩的环境和生理因素。这项研究将是第一批考虑同质性进化机制的研究之一，而不是简单地记录这种进化模式的例子。

Viviparity-live-bearing modes of reproduction, rather than egg-laying-has evolved independently in lineages of many animals. It therefore is a consequence of convergent evolution, with the same feature occurring in taxa that do not share a common ancestor that bears the trait (homoplasy). Viviparity in amphibians has many different expressions of these independent evolutions including retention of the internally fertilized ova in the oviducts with our without maternal nutrition after yolk is resorbed, intraoviductal cannibalism, retention of externally fertilized ova in the skin of the mother's back, and others. This research program will investigate the mechanisms by which viviparity has arisen by comparing 1) features of parental ovaries, oviducts, and skin with those of related species that are egg-layers to elucidate the effects of 'pregnancy' maintenance; 2) patterns of development of embryos of taxa with different modes of viviparity and with related egg-laying species; 3) modes of retention of the developing embryos, including the hormonal substrate as measured morphologically; 4) modes of nutrition of the developing young; 5) the features of metamorphosis of the young of viviparous species as compared with those of related egg-layers; and 6) length of 'pregnancy' and the environmental and physiological cues for birth. The study will be one of the first to consider mechanisms by which homoplasy has evolved, rather than simply documenting instances of such patterns of evolution.